This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 9902128-9 filed in Sweden on Jun. 4, 1999; the entire content of which is hereby incorporated by reference.
The present invention relates to a micro-electromechanical switching arrangement with electrostatically controlled distance between at least a first and a second contact element arranged on a semiconductor layer, preferably provided on a substrate.
More specially, the present invention relates to a capacitive component with variable capacitance, and more specifically to a micro-electromechanical (MEM) capacitor having electrostatically controlled distance between first and second conductive layers arranged on a semiconductor layer, preferably provided on a substrate.
Known micromechanical switches comprise two connector arms which are brought together under influence of a force to make contact and conduct current through the as or other signal carriers arranged on the arms. The arms are usually made of two different materials having different thermal expansion coefficients. When the arms are exposed to heat, they bend because of the different thermal expansion coefficients and come into contact with each other or move further from each other. These types of switches need heating elements to heat the connecting arms.
Varactors are capacitors with voltage dependent capacitances. Semiconductor varactors are based on p-n, p-i-n or similar type of junctions, see for example xe2x80x9cC.M. Cze, Physics of Semiconductor Devicesxe2x80x9d.
In microwave applications, the quality factor (Q-factor) of semiconductor varactors degrades extremely with the increasing frequency due to the microwave losses both in doped semiconductor regions (dielectric losses) and metal-electrodes (conduction losses), Generally, in commercially available varactors the Q-factor is limited by about 10-20 above frequencies about 10-20 GHz and decreases with increasing frequency. Another disadvantage with semiconductor varactors is tat they are highly nonlinear devices, while in some microwave applications, e.g. in tunable filters, a high linearity is required A further disadvantage with semiconductor varactors is that although they operate at reverse bias conditions they have certain leakage currents, which increase with increased temperature or optical illumination (in the case where it is used as an optically controlled varactor).
To improve the quality factor and linearity a micromechanical varactor is proposed, for example in xe2x80x9cDec A., Suyama K., Micromechanical Varactor with a wide tuning rangexe2x80x9d, Electronics Letters, Vol.33, pp. 922-924, 1997. In this varactor no semiconductor or dielectric layers are provided between the plates of the capacitor, resulting in higher Q-factor (limited by conduction losses only) and absence of non-linearity. Moreover, no leakage currents occur in the device according to this document as long as the applied voltages are lower that the breakdown voltage of the air. A schematic cross-sectional view of one embodiment of such a device 1 is illustrated in FIG. 1, The device 1 comprises a first (upper) thin metallic plate 2, a second (lower) thin metallic plate 3, a dielectric substrate 4 and terminals 5 and 6, and a dielectric layer 7. The metallic plates are distanced from each other at a distance dxe2x80x2, which is hxe2x80x2a(v)+hxe2x80x2d, where hxe2x80x2a(v) is the thickness of the air gap, and hxe2x80x2d is the thickness of the dielectric layer 7. One end of the first plate 2 is fixed on a supporting part of the substantially L-shaped substrate 4 and the other end of it projects over the second plate 3 provided on a lower (horizontally projecting) part of the substantially L-shaped substrate. By applying an external voltage through the terminals 5 and 6, charges are generated on the metallic plates of the capacitor, Due to the large elasticity of at least one of the plates, and as a result of the electrostatic attraction force generated through the charges of opposite signs (negative on one and positive on the other plate), the free end of the first plate and the second plate are moved relative to each other.
A variable capacitor similar to the latter is also the subject of the European Patent Application No. 759 628.
The thin dielectric layer is arranged to avoid a short-circuit between the plates 2 and 3. As the result the total capacitance becomes:       C    tot    =      1                  1                  C          a                    +              1                  C          d                    
where Ca is the capacitance of air=(xcex5oS)/(hxe2x80x2a(v)),
Cd is the capacitance of the dielectric layer=(xcex5xcex50S)/hxe2x80x2d,
xcex5 is the dielectric constant of the dielectric layer,
xcex50 is the dielectric constant of air,
S is the overlapping area of the plates,
hxe2x80x2a(v) is the thickness of the air gap, and
hxe2x80x2d is the thickness of the dielectric layer,
The thickness hxe2x80x2a(v) is voltage dependent and consequently the CEOE too. Accordingly, the maximum capacitance (Cmax) is obtained when ha(v)=0, i.e. Cmax=Cd. This means that the dielectric layer effectively reduces the tunability, i.e. the range of charges, of the capacitance. Consequently, the protecting dielectric layer becomes a disadvantage.
Furthermore, the dielectric layer 7 accumulates charge, which deteriorates the varactor performance. Moreover, when manufacturing a varactor, the arrangement of the dielectric layer is an extra moment.
In large arrays of capacitors used in some electronic circuits and particularly in large arrays of varactors (see for example: Drangmeister R.G. et al, xe2x80x9cFully Reconfigurable Microwave Millimeter wave Circuits Using MEMSxe2x80x9d High Frequency Silicon Micromachining and Integration Workshop, MTT-S""98) the number of connector strips required to apply the control voltages to each of the individual capacitors increases with the number of capacitors, resulting in complex and less cost effective designs. Moreover, in microwave circuits the connecting DC strips significantly degrade the performance of the circuits based on arrays of electrically controlled micromechanical capacitors. This becomes a severe problem for MEM arrays used in microwave applications.
Non-micromechanical variable capacitances controlled by illumination are also known. U.S. Pat. No. 3,911,297, for example, teaches a variable capacitance diode comprising: a substrate of semiconductor material, a layer of glassy amorphous material disposed upon the said substrate and forming a diode junction. The layers exhibit different kinds of electronic conductivity. The diode further includes first and second conductive means for making ohmic contact with the said substrate and layer of glassy amorphous material respectively, and a source of controllable intensity light optically coupled to the said diode junction for supplying light to the junction. Means are arranged for varying the intensity of the light from the light source thereby varying the capacitance between said first and second conductive means by changing the characteristics of the substrate. Similar devices are also known through the international patent application no. WO 92/04735.
The main objective with the present invention is to overcome the previously mentioned drawbacks connected with the devices according to the prior art.
One main object of the present invention is to provide a micro-electromechanical device, which can be actuated optically.
Another main object of the present invention is to provide an optically controlled variable micro-electromechanical capacitance, a so-called varactor, in which the conductors connected to an external driving source (e.g. a DC driving source) are eliminated.
Another object of the present invention is to improve the electrical performance of the varactor, provide a highly linear device, with low or no leakage currents.
Still, another object of the invention is to keep the microwave losses low (high Q-factor) in microwave applications.
One further major object of the present invention is to provide a micro-electromechanical varactor, which is less responsive to short-circuit.
Moreover, the device according to the invention is simple to design and fabricate (preferably using conventional fabrication processes), which makes the device more cost-effective.
For these reasons, in the initially mentioned capacitor said semiconductor layer constitutes a voltage generator, which when exposed to a radiation produces a voltage charging said first and second conductive layers and induces said electrostatically generated force. Preferably, said semiconductor layer comprises a high conductivity p+ or n+ type semiconductor layer, a substantially high resistivity n or p layer and a n+ or p+ layer. Alternatively, the semiconductor layer consists of a Schottky barrier, p-n or p-i-n diodes. In a preferred embodiment the first conductive layer is deposited on the high conductivity layer and the second conductive layer is deposited on the high resistivity layer. Moreover, the second conductive layer is insulated electrically from the high resistivity layer by a dielectric (oxide) layer and the second conductive layer is galvanically connected to the n+ or p+ layer through a via, which is electrically insulated from the n layer by means of surrounding walls, which are made of a dielectric (oxide) layer. Preferably, at least said second layer preferably, which is of a dielectric material, is provided with a coating for preventing short-circuit between the plates.
To prevent short circuit between the plates, in one preferred embodiment, an internal resistance between p+ and n+ layers and an internal capacitance of p+-n-n30  structure are provided which result in that when the plates of the variable capacitor are short-circuited, a short-circuit current is generated, which results in a voltage drop basically equal to an open circuit photo voltage resulting in a reduction of the voltage on the plates of the capacitor and accordingly reduction of the electrostatically generated force between the plates. Preferably, said resistance is at least partly varied by varying at least a portion of a cross-section at least one of said layers.
In an advantageous embodiment the semiconductor substrate layer consists of Silicon, GaAs, InP etc. and the entire variable capacitance is arranged on a substrate consisting of metal, semiconducting or dielectric material.
To expose the semiconductive layer to the radiation, said first conductive layer is transparent to radiation or arranged with apertures. An one alternative embodiment, the semiconductive layer is exposed to said radiation from side sections or a bottom section.
Preferably, to minimise the losses of the optical power, the thickness of the p+/n+ layer is chosen to be smaller or comparable to an optical penetration depth of the material at the wavelength of the controlling radiation signal. Preferably, the wavelength, xcex, of the radiation is defined by       λ    =          1.24              E        g              ,
where Eg is the band gap of the high resistivity layer. Most preferably, the radiation is an optical illumination, which has an illumination intensity and said intensity and/or a cross sectional area of an illuminating beam and/or illuminated area is variable. Advantageously, the optical illumination has a CW (Continues Wave) component to set an initiated value of the capacitance, and a variable fraction of the illumination changes the capacitance about a fixed initiated value.
In one alternative embodiment, two semiconducting junctions are connected in series to increase the photo-voltage supplied to the capacitors.
In a low pass filter including a variable capacitance according to the invention and an inductance, the variable capacitance comprising a first conductive layer, a second conductive layer and a semiconductor layer, said fist and second layers being arranged to be displaced relative each other under influence of an electrostatically generated force, and said semiconductor layer constitutes a radiation detector, which when exposed to a radiation produces a voltage charging said first and second conductive layers and induces said electrostatically generated force.
The micro-electromechanical switching arrangement according to the invention comprises a first connector member, a second connector member and a semiconductor layer, said connector members is arranged to be displaced relative each other under influence of an electrostatically generated force. The semiconductor layer constitutes a voltage generator, which when exposed to a radiation produces a voltage charging said first and second connector members and inducing said electrostatically generated force.